That site is so weird. And whoever wrote that article is also mathematically illiterate. Not to mention they didn't link to any sources. So here is one they claim to have used:
This source says that 65% of 25-64 year old spoke at least a second language in 2016. The much lower number of 24% is when asked about proficiency, which can't be compared with the US given Americans' notorious overestimation of their own qualifications.
Most of the 25% of Americans (I couldn't find a source for this) claiming to speak a second language is immigrants. I guess it needs to be said, but when people comment on the monolongualism of Americans, it's about those who are not immigrants or first generation born in the US.
In my experience, most non Hispanics claiming to speak Spanish in the US struggle to hold even a basic conversation. And I have been to 35+ states, including door to door canvassing, etc.
This post is more reasonable and well written than plenty of stuff I have seen from actual right wingers. I worry when people cosplay too hard, sometimes it becomes real.